In 1992 D. A. Fishman suggested that future fiber-optic transmission systems would likely take advantage of fiber amplifiers to increase the distance and capacity of unrepeatered fiber spans, and that they would employ externally modulated, narrow linewidth, tunable CW lasers capable of transmitting signals over long unrepeatered spans even in the presence of large levels of chromatic dispersion. He further indicated that technological advances would be focused on increasing span lengths and capacity by reducing dispersion penalties, improving the optical amplifiers, and addressing limitations due to fiber nonlinearities. See, Journal of Lightwave Technology, Vol. 11, No. 4, pp. 624-632 (1993), which is incorporated herein by reference.
Indeed, fiber nonlinearity and dispersion continue to impose limitations on the performance of fiber-optic transmission systems, particularly high bit rate digital systems. They introduce signal distortion which in turn produces intersymbol interference and reduces the maximum transmitted power, the signal-to-noise ratio, and the unrepeatered span length.
Theoretical analysis by Y. Kodama et al. has indicated that by properly pre-chirping (or pre-distorting) a non-return-to-zero (NRZ) digital signal certain nonlinearities of the fiber (e.g., self phase modulation) and the dispersion of the fiber (a linear effect) may both be canceled. The appropriate pre-distortion depends on the input power level, the type of fiber, and the average dispersion of the fiber link in question. See, OFC'96 Technical Digest, Paper Tul6, pp.48-49 (1996), which is incorporated herein by reference. However, Kodama et al. provided no actual transmitter design to verify their theory, and as a consequence disclosed no actual pre-chirp waveform which would accomplish the reduction in signal distortion. Others in the art have endeavored to address the linear (positive) dispersion problem by, for example, introducing negative wavelength chirp via a suitable bias voltage applied to an external electroabsorption (EA) modulator integrated with a DFB laser. The EA modulator in these designs served a dual function: to introduce into the transmitted signal both negative wavelength chirp and amplitude modulation. See, for example K. Morito et al., IEEE Photonics Technology Letters, Vol. 8, No. 3, pp. 431-433 (1996) and Y. K. Park et al., IEEE Photonics Letters, Vol. 8, No. 9, pp. 1255-1257 (1996), both of which are incorporated herein by reference. These devices are reported to have a chirped NRZ output signal in which the leading edge of the pulses was red-shifted (i.e., slightly increased in wavelength) whereas the trailing edge of the pulses was blue-shifted (i.e., slightly decreased in wavelength). Transmission at 10 Gb/s over 100 km and 130 km standard fiber without dispersion penalty was also reported. However, neither Morito et al. nor Park et al. address the problem of fiber nonlinearities. Moreover, the dual function EA modulator is disadvantageous because the pre-chirping and data encoding processes cannot both be optimized unless each function can be controlled independently. For example, the modulation waveform parameters (e.g., amplitude, voltage-offset and shape) for the highest extinction ratio encoding will typically not be optimized for pre-chirp dispersion compensation.
However, we believe further improvement in transmission, addressing both fiber nonlinearities and dispersion, is realizable if an arbitrary, tailored (e.g., optimized) pre-chirp waveform can be made to pre-distort the output signal of the laser transmitter.